Mesoporous silica, which is porous silica having uniform pores in the mesopore region of 1.5 to 10 nm, clearly shows crystallinity in the powder X-ray diffraction. It is a new material expected as widely applicable to selective catalytic reactions, adsorption/separation, etc.
Usual porous silica materials such as silica gel have hydrophilic surface. In contrast thereto, mesoporous silica has hydrophobic surface.
With respect to processes of the synthesis of mesoporous silica, there have been known, for example, the following three processes with the use of a combination of a silica source with a template. In the first process reported in U.S. Pat. No. 3,556,725, JP-W-5-503499, JP-A-8-34607, etc., the synthesis is carried out within the alkaline region by using amorphous silica powders, aqueous alkali silicate solutions, active silica, etc. as the silica source and quaternary ammonium salts having long-chain alkyl group or phosphonium salts as the template. In the second process reported in JP-A-4-238810, etc., the synthesis is carried out by the ion exchange method with the use of layered silicates like kanemite as the silica source and long-chain alkyl ammonium cations as the template. In the third process reported in U.S. Pat. No. 5,672,556, etc., alkoxides such as tetraethoxysilane are used as the silica source and alkylamines, etc. are used as the template. (The term “JP-A” as used herein means an “unexamined published Japanese patent application”, and the term “JP-W” as used herein means an “published Japanese patent application in the national stage of International application”.)
In the first process, the synthesis is performed within the alkaline region using a strongly cationic surfactant such as a quaternary ammonium salt as the template, so that mesoporous silica is obtained by removing the template. To remove the template, either oxidative destruction in the atmosphere or proton exchange with an acid and a solvent should be performed In the second process, it is further required to synthesize kanemite, etc. used as the starting material. Although the third process with the use of an amine is advantageous from the viewpoint of removing the template, it is inadequate for mass production on an industrial scale, since expensive alkoxides (tetraethoxysilane, etc.) should be used therein.
To synthesize mesoporous silica, therefore, it is unavoidable either to perform the reaction in the alkaline region with the use of strongly cationic surfactants (quaternary ammonium salts, etc.) as the template or to use expensive starting materials such as alkoxides. The mesoporous silica synthesized under these conditions has hydrophobic surface, which makes it disadvantageous in absorbing aqueous waters, etc.
With the spread of Internet and digital cameras, opportunities to output rich full-colored images on paper, etc. have been increasing. Ink-jet printers have been rapidly spreading as instruments for outputting these images owing to the merits thereof such as being able to easily give full-colored images, being available at a low cost, and making little noise. In the ink-jet system, ink droplets are jetted from a nozzle at a high speed and adhered to a recording material to give a record. Since droplets of ink containing much solvents are continuously jetted, there frequently arise some problems such that the ink droplets are fused with each other on a recording sheet to thereby form enlarged dots or mixtures of dots with different colors. It is therefore required that an ink-jet recording sheet can quickly absorb inks so that the inks are neither mixed with each other even in overlapping dots nor blur. It is also required that the recording sheet can keep the image to be recorded in a favorable state (i.e., having excellent water resistance, light resistance, etc.).
From these points of view, it has been proposed ink-jet recording sheets produced by applying various organic matters or inorganic matters optionally together with binders onto base materials or incorporating these matters into base materials. For example, there have been known recording sheets provided with an ink receptor layer made of a water-soluble resin (polyvinyl alcohol, etc.) on paper, a plastic film, etc. and recording sheets provided with an ink receptor layer containing a filler such as silica gel (see, for example, JP-A-55-146786, JP-A-56-99692, JP-A-59-174381, JP-A-2-276670). However, there has been obtained no recording sheet so far which is free from dot blurs, shows a high ink-absorptivity and, at the same time, has satisfactory water resistance and light resistance.
The present invention provides mesoporous silica having highly hydrophilic surface compared with the conventional ones and a process for synthesizing the mesoporous silica under mild conditions with the use of inexpensive materials.
The invention also provides ink absorbents, ink absorbent slurries and recording sheets which are free from dot blurs, have a high ink-absorptivity and are excellent in water resistance and light resistance.